Microchips are formed from a variety of dissimilar materials. A semiconductor die, formed from silicon, gallium arsenide, germanium, or some other semiconductor material may be attached to the flag portion of a heatsink. Typically the heatsink is formed out of copper. The semiconductor die is bonded to the copper with a typical die bond soldering compound. The heatsink is attached to a lead frame made from a metal. The lead frame includes a series of lead contacts that serve as the output electrical contacts for the microchip. To electrically couple the semiconductor die to the lead frame, metal wires extend between the semiconductor die and the lead contacts. To protect the semiconductor die, wires, and lead contacts, a plastic mold compound encapsulates the semiconductor package.
During normal operation, the microchip will go through cycles of heating and cooling. These heating and cooling cycles cause the microchip to expand and contract. Since the microchip is made from dissimilar materials, the expansion and contraction rates within the microchip are not uniform. The different materials forming the microchip expand and contract at different rates. These differing rates of expansion and contraction within the microchip can lead to high internal stresses that can result in device failure. Primarily, the plastic mold compound expands at a sufficiently different rate such that it can separate from the metal heatsink and metal lead frame. This separation is referred to as delamination. When the plastic mold compound delaminates from the heatsink and lead frame, significant stresses are placed on the metal wires that couple the lead contacts to the semiconductor die. These stresses can become high enough such that it breaks the electrical contact between the wires coupling the lead contacts to the semiconductor die. Delamination can also cause microchip failure through moisture and oxidation. If the plastic mold compound delaminates in a manner such that a path from the outside world is created to the semiconductor die and wires, moisture can attack the wires and semiconductor die. Through corrosion, the moisture can cause the microchip to fail. It therefore becomes highly desirable to develop methods to preserve the integrity of the microchip and prevent delimination.
Chemical bonding is one method of adhering the plastic mold compound to the heatsink. The plastic mold compound is formed from an epoxy resin that has a composition that chemically bonds with the native oxide layer of the copper heatsink. While this chemical bonding does adhere the plastic mold compound to the heatsink, successive heating and cooling cycles can break these chemical bonds causing the plastic mold compound to separate from the heatsink.
Altering the chemical composition of the plastic mold compound to give it a rate of thermal expansion identical to the metal heatsink is another way of addressing the delamination problem. Minimizing the difference between the expansion and contraction rates of the metal microchip components and the plastic mold compound reduces the problem of delamination. If the metal components of the microchip and the plastic mold compound had identical rates of thermal expansion and contraction, the chemical bonds between the plastic mold compound and the metal heatsink would not experience mechanical stress from the thermal fluctuations. At this time, a reliable plastic compound with such properties remains unknown to the art.
Another method of addressing the problem of delamination is through locking and bonding the heatsink and plastic mold compound in such a manner that they mechanically interlock. The structures that interlock the plastic mold compound and the heatsink are commonly referred to as mold locks. Mold lock designs currently known to the art have a size and design that require their placement outside of the die attach area of the heatsink. Mold locks currently known to the art are usually formed in a trench-like moat design surrounding the flag portion of the heatsink. Placement of currently known mold locks in the flag portion of the heatsink leads to a variety of problems. If known moat-like mold locks are placed under the semiconductor die, the mold lock acts as a drain for the solder used to bond the semiconductor die to the heatsink. As a result, known mold locks degrade the bond between the semiconductor die and heatsink by draining the solder away. In addition, through draining solder away from the semiconductor die heatsink interface, known mold locks degrade the chemical bond between the plastic mold compound and heatsink when they are placed under the semiconductor die. As discussed above, the plastic mold compound has a composition such that it forms a chemical bond with the oxide layer of the copper heatsink. In contrast, the plastic mold compound does not form a chemical bond with the solder compound used to attach the semiconductor die. When existing mold locks drain solder away from under the semiconductor die heatsink interface, the area of the plastic mold compound heatsink interface is decreased and the area of the plastic mold compound solder interface is increased. Since a chemical bond is not formed between the solder and plastic mold compound, the overall bond between the plastic mold compound and the heatsink is degraded when known mold locks are formed under the semiconductor die. Still further, known mold locks adversely impact the bond line thickness.
The size of the semiconductor die varies depending upon the application for which the die is designed. At present, the semiconductor industry manufactures heatsinks and lead frames that have a size and design that match the size of each individual semiconductor die. Designing and manufacturing custom heatsinks and lead frames to match each size of semiconductor die is an expensive process. The semiconductor industry is constantly searching for methods to reduce the cost of manufacturing microchips. One method of reducing cost is through designing a “one size fits all” heatsink and lead frame structure that is compatible with a variety of semiconductor die sizes. In designing this “one size fits all” heatsink and lead frame structure, it is highly desirable to develop a mold lock structure that preserves microchip package integrity when used in conjunction with both large and small semiconductor die sizes in order to reduce cost and design time.
Designing a “one size fits all” heatsink structure presents a variety of challenges for mold lock design and microchip integrity. To view these design challenges, examine the integration of a large semiconductor die and a small semiconductor die with the same heatsink structure. When integrating the large semiconductor die with the heatsink, the plastic mold compound will see the mold locks formed on the exterior edge of the heatsink outside the die attach area and the semiconductor die itself lying in the flag area. However, when a small semiconductor die is placed on the same heatsink, a large bare area of the die attach area is exposed to the plastic mold compound. This large area of the flag has no mold locks to secure the plastic mold compound due to the difficulties known to the art discussed above. Consequently, the semiconductor package having the small semiconductor die and “one size fits all” heatsink is now at risk for delamination. It is therefore highly desirable to develop a mold lock structure that is useable in the flag portion of the heatsink that addresses the problems known to the art. In this manner, it is possible to develop a “one size fits all” heatsink structure that preserves the package integrity of the microchip.